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What does the autopilot do if activated in a stall condition?
I was thinking about Air France 447 recently and had a wonder:
What would happen if the pilots had re-engaged the autopilot and stopped all stick input? (If that's even possible in a stall condition). I know this has since changed, there is the MCAS system now, which will force a stall recovery. But in an older aircraft, like the A330 - 203, what would happen?

MCAS on the 737MAX is not an anti-stall system.

There is a very basic flying skill issue here. A brand new pilot is taught not to be dependent on airspeed indication, doubly so for an IFR pilot. You have speed, pitch attitude and altitude.  If airspeed goes wonky, you look to see that the pitch attitude is also doing things that would match the speed indications. If the speed indication is dropping, but the power is on and the pitch attitude is constant, and you don't feel any deceleration, those things outvote the airspeed indication and you just hold the pitch attitude.The two guys on 447 suffered a very basic skill breakdown.

In the AF447 accident, the autopilot disconnected because of inconsistent airspeed:

Temporary inconsistency between the airspeed measurements, likely following
the obstruction of the Pitot probes by ice crystals that, in particular, caused the
autopilot disconnection and the reconfiguration to alternate law;

(BEA - Final Report)
As long as the airspeed remains inconsistent, it is not possible to re-engage the autopilot:

Two ADR outputs are erroneous but different and the remaining ADR
is correct, or if all three ADR outputs are erroneous but different. The
AP and A/THR will disconnect. If the disagreement lasts for more than 10
seconds, the PRIM triggers the NAV ADR DISAGREE ECAM caution.
Flight controls revert to ALTN 2 law. The SPD LIM flag is displayed on
both PFDs, however VLS and VSW are not displayed. This condition is
latched until a PRIM reset is performed on ground without any hydraulic
pressure. However, if the disagreement was transient, the AP and A/THR
can be re-engaged when the NAV ADR DISAGREE message has
disappeared.

(Airbus A330 FCTM - Non-normal operations - Unreliable Airspeed Indications, emphasis mine)
It is not known if the crew attempted to re-engage the AP with the pushbuttons on the FCU because this is not recorded by the FDR:

It is also impossible to see whether there have been any attempts to re-engage the autopilot.

(BEA - Final Report)
The BEA did a study of similar incidents and found that the AP was re-engaged in some of these:

The BEA has studied thirteen unreliable indicated airspeed events involving the
temporary loss of this reading, or other anomalies, for which it had access to crew
reports, recorded parameters and PFR. [...]

The aircraft’s autopilot disconnected in all cases, with no intervention from
the crew;
[...]
In seven cases, an autopilot was re-engaged during the event. In two of these,
the re-engagement occurred even though two speeds were consistent with each
other, but erroneous;

(BEA - Final Report)
So it is possible to re-engage the AP after it automatically disconnects due to unreliable airspeed, but these events were not fully developed stalls. Your question seems to ask about the situation later on where the incorrect manual inputs had already resulted in a stall. In these cases, re-engagement of the AP would likely not be possible due to the engagement conditions:

AP Engagement
AP can be engaged when:

Aircraft speed is within VLS and VMAX.
Aircraft pitch angle does not exceed 10° nose down or 22° nose up.
Bank angle is less than 40°
[...]

(Airbus A330 FCOM - Auto Flight - Flight Guidance - Autopilot)
In a stall the speed or pitch limits are likely exceeded preventing autopilot re-engagement. This is a part of the FDR recording from AF447 (you can see that pitch never exceeded the 22°, but the airspeed went down to 38 kt towards then end of the flight while the F/O pitch command was at the nose-up limit):

(BEA - Final Report)
If the autopilot would re-engage successfully, it will do what it is programmed to do: follow the flight director inputs, which depend on the selected mode on the FCU (therefore just turning on the AP is not enough, one also needs to select appropriate modes on the FCU).

Outstanding answer!

Note that the aircraft's actual airspeed was much higher than 38 kt; the aircraft's extreme angle of attack disrupted the airflow over the pitot tubes, producing falsely-low airspeed indications.  (This is not to be confused with the earlier erroneously-low airspeed indications resulting from pitot-tube ice-crystal blockage, which had long since cleared by this point.)

@Sean Agreed, but the autopilot engagement condition is obviously based on that speed, not the actual (but unknown) speed.

@Bianfable: I know - just noting in case anyone thought you were saying that the aircraft's airspeed actually went down to 38 kt.

On just about all airplanes the Stall Protection System disengages the autopilot automatically whenever a range of parameters is exceeded.  One of them is approaching stall.  Generally the Autopilot will be kicked off automatically just before stick shaker onset on a traditional non-FBW airplane, and it can't be reengaged until the parameters that were exceeded (pitch/bank angles, speed, AOA etc) are back in limits.
Low speed flight at high altitude is dangerous because the recovery process requires extreme inputs to recover speed compared to down lower (mass is the same, but the air is thin) - you have to dive aggressively to gain speed so even if you could, just reengaging the AP would be inadvisable.
Since you mentioned it, I want to repeat an important point on MCAS because there appears to be a mountain of misinformation. MCAS has nothing whatsoever to do with stall recovery or stall behaviour.
It was implemented strictly to make the MAX's pitch stick force response in certain regimes match older 737s, to avoid an additional training requirement necessitating Sim time. That's ALL it was intended to do.  The airplane was technically certifiable as it was when the problem was discovered (it wasn't dangerous, just different), but would have required sim training, which was considered a marketing disaster from an airline cost perspective.

Hi John, thanks for correcting me! Can I ask, why don't they program autopilots to recover from a stall? So for example in AF447, they could re-engage AP, which would then automatically perform a pitch down and level off once correct airspeed was achieved? Seems like that would be a lot safer than the human instinct to "climb"!

Autopilots have traditionally not had control authority to do those things. They are designed to maneuver the airplane in normal ways, the way a smooth pilot would. The aren't designed to take over and fly beyond gentle turns and pitch rates. It's expected that a pilot who knows how to fly can take care of that. An autopilot that can do what you suggest is now becoming a "robot pilot" you might say, with all of the powers of the human pilot.  If the "robot  pilot" gets out of control, you have a problem. This may happen in future if somebody ever tries to certify a single pilot airliner.

Or... a no-pilot airliner :0

@Cloud I wouldn't call what happened on AF447 "human instinct," at least not for a trained pilot. Every competent pilot is trained from very early on to almost reflexively drop the nose at the first hints of the onset of stall. This will be the natural reaction for any competent pilot. Why (at least) one of the pilots on AF447 was not competent is another question. Maybe it's a human instinct to pull back when you see the ground approaching too quickly (e.g. Asiana 214,) but, at that point in an airliner, it's more a matter of determining how you're going to crash than actually preventing it.

A correctly functioning autopilot does not need a stall recovery procedure for the same reason that a correctly functioning human pilot does not need one: Neither would put the plane in a situation where a stall was going to happen. Of course human pilots are taught to recover from stalls because (1) they sometimes make mistakes and (2) sometimes, automatic systems do not function correctly.

It seems to be popular to imagine (probably because of the name) that an 'autopilot' can do a lot more than it's actually capable of. In reality it somewhat resembles a car's cruise control. Most of the time it's maintaining a constant course at a constant speed and altitude (in conjunction with the autothrottle). Nothing particularly clever. Sure it can change course, speed or altitude but does so within strict limits.


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What programming language(s) is Boeing's MCAS written in?
What language is Boeing's MCAS written in?
Thanks

The answer to this question may help: https://aviation.stackexchange.com/questions/3608/what-programming-languages-are-used-for-equipment-onboard-aircraft I'm guessing Ada in a safety critical subsystem like that.

As long as it is not assembler, I doubt the language itself was the main reason of the failure. I would even doubt be it assembler, if done properly, with full test coverage and code reviews.

Why has this question been downvoted? - I'd vote to reopen but have insufficient rep. This is not only a reasonable question, but an interesting and important one.

@stevec There are a few "troll" users on here, who down-vote every question I ask without discrimination. I have stopped using the site mostly for that reason

@Cloud don't stop, your question is a very good one. Funnily, I found my way back to it when researching the 737 max a second time. Thank you for your question, even if others don't appreciate it

I don't think I down-voted this question, but I can understand why some might have done so. You don't tell us why you want to know, or in fact, give us any more information other than simply restating the exact wording from the title to the body. You don't give any information about prior research you have done, which means, any potential answerer may re-do work you already did. So, let's look at the hover-text from the down-vote button: This question does not show any research effort. True. It is unclear False or not useful Partially true. Why do we care what language?

You want us to research your answer for you, yet you have not taken time to tell us why, or shown us that you have done even a minimal amount of research before hand. This does not explain why it was closed, but does explain why it has so many down-votes.

Fair enough @CGCampbell thanks for the feedback!

What language is Boeing's MCAS written in?

I believe the Flight Control Computer is a Collins Aerospace FCC-730 with two 16-bit processors.
Collins are known to use Ada for at least some of their products.
A job advert for Collins stated:

Job description: Sr Software Engineer - Flight Controls ...
Required Qualifications: ... Experience with C/C++ ...
Preferred Qualifications: Experience using Ada. ...

Ada is widely used in the aviation industry for flight-critical applications.
Other Boeing aircraft make extensive use of Ada:

in the 777 program it was a goal to use Ada as a standard language, and it represented nearly 70% of the source line of code developed for the 777.

So it is very likely that they used Ada for the code running in the FCC-730 on the 737 Max.

Max Software Hardware
Rockwell Collins ...port the existing Ada codebase - display not flight control.
Sr Software Engineer - Flight Controls
Boeing 777 Flight Control System

source: Stack Exchange

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What happens if an ATC tower is completely disabled?
What is the procedure for aircraft, if the ATC they were relying on to land, is, in some way, completely disabled? (for example there is a fire that destroys the whole building and all equipment).
If this happens at a busy airport, do the aircraft simply communicate with each other to organise who will land first etc.?
For the purpose of the question, assume there is no other tower within range that could reasonably pick up the job and that there are aircraft that must land there (they might have already been in holding for 2 hours plus and have no fuel left).
This is not a duplicate as I am specifically asking about cases where they have not been diverted to another airport, are in holding, run out of fuel and then ATC tower gets disabled.

@MichaelKjörling Not quite, I've added explanation to the question.

See also How are landing aircraft handled during an emergency runway closure?, particularly J. Hougaard's answer and the comments to that. Your question appears to posit a situation that simply would not occur if the pilots involved are doing their jobs, including following regulations that could get their licenses suspended or revoked if they don't.

If we assume that the tower is somehow out of commission and the FAA can not get a temporary facility up there are a few options.
They could close the runways with appropriate markings and a potential update to the ATIS (if its still operational). They could coordinate with local ATC facilities to help with the diversions as well.

(source)
Perhaps the simpler solution is that the airport becomes uncontrolled. This is what happens at my local field KPNE late at night. Not all towers are always operational and some shut down at night. In the case of KPNE the tower is only operational 0600-2300L, outside of that the airport becomes class E airspace and is considered uncontrolled and airplanes operate as such. In a situation where the runway is not closed and there is no tower aircraft are perfectly capable of coordinating themselves.

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How do I get rated to fly complex aircraft that are still Microlights?
In the most recent issue of the BMAA magazine, there are new aircraft listed that are still Microlights, but have complex features. These include Autopilot(!), retractable landing gears and even variable pitch props.
If one was to look at purchasing such an aircraft, how do ratings work and which body endorses them in the UK (if you hold an NPPL only)? Do you need separate training for each complexity, i.e. a retractable landing gear course, or is there one that covers them all for Microlight pilots?

Are you asking specifically for flying on a UK NPPL, or more generally?

@Jamiec good point, have edited the question :)

It appears that the British Microlight Aircraft Association is who you should refer to. They have a document on Differences Training. Details of all differences training can be found therein, but some of your specific questions pulled out...
Autopilot is mandatory:

Before exercising the privileges of the rating, the holder must complete appropriate differences training, if the
aeroplane is fitted with Autopilot System
and the holder does not have training or experience in aeroplanes with such fitted features. The differences training
mentioned above must be given by a flight instructor entitled to instruct on the aeroplane on which the training is being
given, recorded in the holder’s personal flying logbook and endorsed and signed by the instructor conducting the
training.

Retractable u/c is "advised" (Egad! This is the one you don't want to get wrong!):

Although not specified in the August 2021 ANO update as requiring differences training by law, landing with the
undercarriage retracted is one of the most common accidents in complex light aircraft. Light aircraft training typically
includes a “gear down” check as standard in circuit and before landing checks. All precautions should be taken to avoid
this sort of accident; several broken aircraft each year demonstrate the importance of this training.

Variable pitch prop is mandatory:

Before exercising the privileges of the rating, the holder must complete appropriate differences training, if the
aeroplane is fitted with a variable pitch propeller
and the holder does not have training or experience in aeroplanes with such fitted features.
The differences training mentioned above must be given by a flight instructor entitled to instruct on the aeroplane on
which the training is being given, recorded in the holder’s personal flying logbook and endorsed and signed by the
instructor conducting the training.
Comment: For clarification, a variable pitch propellor (VPP) is one whose angle of attack can be adjusted in flight. Many
microlights have ground adjustable propellors whose angle of attack can only be adjusted on the ground; these are
not included within this differences training requirement.
Mishandling of a VPP will likely lead to engine damage with the safety risk that it brings and to extremely expensive
repair bills. The differences training will consist of ground training to understand the workings of such devices and
flight training to practice the practical use of such systems, failure modes and emergency procedures.

Is there one "course" which covers all differences you might be interested in is probably a general "no", however if you have access to such an aircraft which has all the features you're interested in and an instructor/examiner qualified to carry out such differences training I'm pretty sure they would do them all in one go. Not a microlight, but that's how I added retractable & complex (basically: Constant Speed Prop) endorsements to my SEP rating on my PPL.

Thanks Jamie! Autopilot sounds so fun!

@Cloud Not so much fun, hand flying is fun. What an autopilot does is removes a huge amount of workload in the cruise allowing you more headspace for other things. Mine is currently inop, and while I miss it on a long straight cruise, it's much more fun actually flying the plane.


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What Caused the Engine-Failure of Southwest Airlines Flight WN-3472?
What caused the damage on Southwest Airlines Flight WN-3472?

The inlet section of the engine is missing but the fan and nose-cone are still visible.
Pictures taken during flight even seem to show the fan windmilling.

The currently known details are that it was caused by the failure of a single fan blade, due to a fatigue crack. The crack location was between the blade root and tip (the root of the blade stayed in the disk).
As the question states, the accident was in a Southwest Airlines Boeing 737-700. The Aviation Herald says here that it was:

...performing flight WN-3472 from New Orleans,LA to Orlando,FL (USA) with 99 passengers and 5 crew, [and] was climbing through FL310 out of New Orleans, about 80nm west of Pensacola,FL (USA) when the front section of the left hand engine (CFM56) separated, debris impacted and punctured the left side of the fuselage causing a loss of cabin pressure. The crew diverted the aircraft to Pensacola for a safe landing on runway 17, the aircraft vacated the runway and taxied to the apron with emergency services following the aircraft. There were no injuries, the aircraft sustained substantial damage.

It also says:

[In] late Sep 12th 2016 the US NTSB reported, that one fan blade separated from the fan disk during the accident flight, the root of the fan blade remained in the fan hub, however, the rest of the fan blade was missing. The fracture surface of the titanium alloy fan blade, root covered by copper-nickel-indium alloy, showed evidence of a fatigue crack, however no surface or material anomalies were found during initial metallurgic analysis. The engine inlet separated from the engine and impacted fuselage, wing and empanage resulting in a 5x16 inch hole above the left wing.

I expect that's the extent that is publicly available at the moment, because the Aviation Herald report would have been updated if further info was available. So, it's not publicly known if the fatigue crack was low cycle fatigue or high cycle fatigue.
Low cycle fatigue is fatigue caused by large changes in stress amplitude, such as caused by a change in rpm from idle to max speed. Because the change in stress amplitude is large, it can fail in relatively few cycles, compared to HCF. (But still maybe ~10,000). Because it's driven by throttle movements, the number of cycles can be counted. Typically a single flight, even ~10 hours, in a civil aircraft only accumulates less than 10 cycles.
High cycle fatigue is fatigue caused by very small changes in stress amplitude, such as turbulence acting on a blade, every time a rotating blade passes behind a stationary vane. Hence, if a stage in a compressor has 30 vanes, every revolution sees 30 stress cycles on each blade in the following stage. Thus, the number of HCF cycles can accumulate very rapidly. They cannot easily be tracked. Usually, HCF failures need some sort of stress concentration to exist at the point of failure, such as a material anomaly, maintenance damage, or a nick from FOD (foreign object damage from an ingested stone from a runway, for example). HCF failures can require 100,000s of cycles to be accumulated, but if you get 30 every revolution, and the shaft is doing 7,000 rpm in cruise, these accumulate very quickly. Initiation to failure can be just a few flights.
Since there were no material anomalies found on the fracture surface of the blade half they have (the outer piece was not recovered), that indicates the cause of the fatigue crack is unlikely to be a manufacturing issue, and hence a fleet-wide issue.
It is probably an HCF failure, since the crack location does not seem to be in the blade root. Usually, the location of the weakest points due to LCF are in the blade root or fir-tree base, and it sounds like it failed higher than that.
What About The Cowling Failure?
When the fan blade fails, it's pretty violent. Here's a video of a blade-off test done for certification. There is very high vibration at the time of failure, and high vibration while the engine is windmilling before landing. Fan blade failures should not result in cowling failure, because the whole point of the test is to demonstrate that the failure is contained. That means, no part is released at high energy, that may damage the aircraft. Having the cowling fall off, doesn't really align with that philosophy. But, clearly, there is a random element in such an energetic failure.
The most receny fan blade failure I believe was an AirAsia X A330-300 off the west coast of Australia, on Jun 25th 2017. It lost almost the entire blade, but the cowling remained very undamaged, as shown in the picture at the link.
The A380 Air France engine failure that occured in Sept 2017 had similar damage to the aircraft in the question, but the Air France incident was not a fan blade failure. The Airworthiness Directive issued after this states the incident was caused by failure of a fan hub. So, it's not a relevant comparison.
Some aircraft have suffered strutural failings of the fan cowl, without being caused by fan blade failure. Eg. This Egypt Air A330-200 and this China Eastern A330-200 both lost a portion of their cowling. But these failures are a hole blown in the side of the cowling, not loss of the entire duct forward of the fan. These were caused by a structural defect in the composite duct.

so do I get this right and the unbalanced rotor (since it missed one blade) shook so much the inlet section disintegrated?

sorry, misread your comment completely, I'll delete it.

The issue with these static tests is they do not accurately simulate the effects while in flight. You can see the cowling is already under severe stress in the video, now add in the airflow of the thing going through air at 550kts plus and it does not take much imagination to picture it ripping off.

@Trevor_G. Correct, the static test is different to inflight. In the static test, the static pressure in the duct is less than ambient pressure, in flight it is greater (i.e. there is a ram effect in flight). But either way, designing and testing the engine to ensure the fan blade is contained if it fails, and then allowing or ignoring structural failure of the cowling, isn't a consistent approach to safety. The cowling is huge. What would have happen if it hit the tail plane? I don't know of another fan blade failure that has caused a cowling failure like this. If you do, I let us know!

The question now is what caused the fatigue failure of the blade ?

@BambOo. Yes, you and everyone is now asking this! I'm not aware of any more info to add, the NTSB haven't released a final report for the above event. I see quite a lot of comments in the 2018 incident on Aviation Herald suggesting something needs to be done to try and stop the cowling separating. This seems to be the item that in both cases penetrated the fuselage. See: http://avherald.com/h?article=4b7725fb&opt=0

@Penguin The damages to the fan cowl are very impressive. Interestingly, the blade-off seems to have been contained by the fan ring, while the cowl itself shattered into pieces. But I can't figure the broken blade to be the cause of such damages. So I guess it must be the dynamic levels on the cowl due to the impact/rubbing of the blades (probably) coupled with the aerodynamic excitation that led to the desctruction of the cowl.

@BambOo. Agree, I can't figure that bit out really either. The pic on AH shows the remaining fan blades and the blade track in normal looking  condition, aside from the missing blade.

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Why is the DC10/MD11 center engine placed where it is?
While reading this question about trijets I started wondering why the center engine in the DC10 and MD11 where placed where they are. Most trijets have the center engine at the very back of the fuselage with the intake in front of the vertical stabilizer. The DC10/MD11 engine is placed above the fuselage at the bottom of the vertical stab.
I'm wondering if there is some advantage to that. Does placing the engine there create an appreciable pitch down moment? And how would that relate to the pitch up moment of the underwing engines? Are there any other aircraft that have this configuration?

Very related: Why is the nacelle outlet of the tail engine of the L-1011 Tristar lower than the engine itself?

There are a few ways the aircraft engines can be mounted on the centerline, each with its advantages and disadvantages.
Image from Stanford University.
One way is to mount them as in DC10, with engine above the fuselage; another one is to use a 'S' shaped inlet and put the engine at the aft of the fuselage like Dassault Falcon 7X.
The main advantage of mounting the engine as in DC10 is that the inlet distortion is minimized and there is no need to cut a hole through the fuselage for putting engine. The main disadvantages are that it increases drag (slightly) and engine maintenance is a mess.
Apparently,the DC10 engineers tried to put all three engines in the wing during design, but were not successful.
Engines mounted in high thrust line like DC10 and Piper PA47 are highly stabilizing as increase in engine thrust causes a pitch down motion as the thrust is increased. This is usually disconcerting to the pilots, who expect the aircraft to pitch up while engine power is increased.
Finally they ended up adding design changes to correct this in PA47 (to make it pitch neutral).
In DC10, a combination of underwing and high thrust line engines would've most probably caused no pitch changes at all. So I guess the main advantage is reducing inlet distortion.
As MD11 was developed from DC10, it simply inherited this design.
Note: As for the pitch down moment being disconcerting to the pilots, it is better if someone corroborates it. It is simply that most aircraft don't do this.

Are there any other aircraft that have this configuration?

For a similar engine configuration, see the Britten-Norman Trislander, a development of their successful twin Islander.

Britten-Norman Trislander (picture source)
Generally, this configuration is unusual. It was driven by the desire to use existing engines on aircraft which grew too large to work with only two engines. Only one other airliner used this layout, the Lockheed Tristar.
Technically, the different location of the engines allows to control pitch with thrust input only. However, the slow response of turbofans makes them unsuitable for maneuvering; only trim support is realistically achievable.

The configuration showed on the picture (the Trislander) looks like the one on the Orlyonok (T-tail, motor in front of the elevator)

@ManuH: Yes, this configuration is also used in a proposed amphibian from France. But both have no wing engines, so they are not exactly the same.

Another aircraft with a tail-mounted engine.

@Meower68: and now imagine you have some imbalance on that tail engine. Shudder!

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Does distributed propulsion create unwanted lift at cruise speeds, requiring a negative angle of attack?
Does distributed propulsion create unwanted lift at cruise speeds?
The great thing about slats and slotted flaps is that you can "turn them on" when you need high lift at takeoff, and "turn them off" when you don't need high lift during cruise.
So what about distributed propulsion by using 4 engines with props covering the majority of the wingspan? You can't turn off the extra lift off if you're trying to cruise as fast as possible.
I understand distributed propulsion adds a clmax of about 3.
Typically, you only need a Cl of 0.5 during cruise, but most GA planes cruise at about 75% power, almost at full power as on takeoff, so wouldn't a cl total of say 1-2 be generated with distributed propulsion?  That's 200 to 400% higher lift than needed.
To get rid of this extra lift and drag, wouldn't you need to lower the nose until the wings produce a " negative induced cl" of about -.5 to -1.5, adding the "distributed propulsion cl" of 1-2, for a total desired cL of about 0.5?
Is this correct, at least conceptually? Or does lift due to distributed propulsion trend to zero at a zero AOA? i.e. it's just additive if the wing is producing lift above a zero AOA

Note that it's clmax, not cl you're talking about.

Why all the references to clmax instead of cl?  Something is wrong with this question.

@Fred you are correct in questioning the effects of prop blast on total lift, but need to consider magnitude.  For a very light model with a high lift wing and excess power, it might be quite dramatic.  Also, consider mounting props ahead or behind the wings (or elsewhere).  Remember, prop wash is going to be turbulent (even if counter rotated) or, as was the case with the B-36, the wing interferes with the prop airstream, which will affect thrust production (and gave the plane an audible pulse that was heard from many miles away). Some prefer to keep the wing "clean".

@Fred just another light bulb about (perhaps the lost art) of propwash: it may actually keep the local AOA of the airstream more constant (from its flow) and control lift simply by adding or removing throttle.  This may help stall proof the wing to a greater degree.  You may wish to look into the Navy "flying flapjack", they also used propwash to counteract wing tip vorticies.

The fundamental way distributed propulsion adds extra lift is by blowing additional air over the wings. This is a direct thrust-to-lift conversion.
If you produce less thrust, you produce less lift with this arrangement.
So, no, you don't go to a negative AoA to counteract the excess lift produced by running all of your motors at full power. You never produce that excess lift in the first place. This involves running your wing propulsors at reduced power, and if you have lift-assist propulsors, such as blown flaps, turning them off altogether.

I'm talking about just using 4 normal airplane engine as a "propulsor". At a full speed cruise, you are producing almost maximum thrust with your props along the entire wingspan, so they would be producing almost maximum distributed lift, no?

@Fred Not quite. Cruise thrust tends to be in the range of 15% to 30% of takeoff thrust.

Maybe for commercial aircraft, but I'm talking about GA aircraft or ultralights, where you cruise at 75% of full power, so you would be still generating 75% of full thrust in cruise, and generating all sorts of extra lift, no?

You don't cruise at 75% of full thrust. You may cruise at 75% of currently available power, but that produces much less than 75% of takeoff thrust.

Wouldn't running negative AoA cause induced drag in the other direction?  Why would you want to have the propulsion system and wing working in opposition to each other?  Seems like an integrated design minimize induced drag from the airfoil to take advantage of the effects of distributed propulsion would be the best option.  Or are you retrofitting an existing design?

@ Therac: With a fixed pitch prop, isn't thrust just a function of what rpm my prop is spinning at? So if my prop is spinning at 2,400 rmp on the ground or 2,400 rpm at 1,000ft AGL, wouldn't it produce the same thrust?

@Fred, prop efficiency drops off as density decreases.  Less air molecules to get traction on.

@ Michael Hall: Well, let's say 50' off the ground, same air density, out of ground effect., isn't the thrust the same?

@Fred It's not. The faster you go, the less thrust your prop produces. For propeller aircraft, it decreases from 3.5-5 lbf/hp at standstill to 1-3 lbf/hp at cruise.

@Fred, 50' is negligible.


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Do wings on a plane need to be in the same plane horizontally, or can one wing root be 6" lower than the other?
Do wings on a plane need to be in the same plane horizontally, or can one wing root be 6" lower than the other?
The proposed application would be for a ultralight with high mount folding wings, where first 50% chord flaps would fold down 180 deg underneath the wings, reducing the "folded chord" to 50% of the full wing chord.  Then the tapered wings would scissor back about 120 deg over the fuselage and interlock like a scissor, so when the wings are folded up, they are roughly mirror the same shape as the fuselage.  One wing root would be about 6" higher than the other.
For example, with a 12" high spar root, and 3" thick wings, one 1/2 span would be mounted near the top of the spar, the other one, at the bottom on the other side of the fuselage

"50% flaps" means that the flaps' "width" is half of the wing chord, not that the pilot has commanded the flaps to be deflected halfway, right?

The left and right wing may be at different heights.  You'd suffer from interference drag at the joint between them, or extra induced drag from the two extra "wingtips" if there is no joint, but a slow ultralight can tolerate such drag better than most airplanes.  This may be the only asymmetrical design that Rutan didn't attempt.


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If you make up your own IFR waypoints, how does ATC know where they are?
If you make your own IFR waypoints, how does ATC know where they are?
Just finished my IFR course in Canada.
Was surprised to learn that you can make up
your own IFR waypoints.
If all IFR flights must register a flightplan, how does ATC know where your waypoints are if you make them up yourself, or do waypoints need to be registered first?
My flying club has their own IFR waypoints and I heard that some of the fly in fishing camps have their own IFR waypoints.

If you make up your own waypoint, don't you give the coordinates?

Are you allowed to name them at will? Or are they waypoints like "N334455/E0123456", which encode the position in their name?

Can you give an example of such a waypoint?

Yes, you can define your own waypoints. When we use the waypoint identifiers that are published ATC knows where the waypoint is. That's because we're using the same database they are which contains the Lat/Lon location for each of the waypoints in the database.
When we create our own waypoints, ATC doesn't know anything about them. So we just have to give them the waypoint in a form they can interpret.
Almost all FMS provide the capability for pilot-defined waypoints. They can be entered in the form of Lat/Lon, Place/Bearing/Distance (PBD), or Place/Bearing-Place/Bearing (PBX). Once created, the system assigns a short ID to be used in the flight plan leg list. Usually something starting with LLxx, PBxx, PXxx where xx is a 2 digit number. Some GPS navigators just have one pilot-defined list of WPTxx.
While these identifiers work well in your FMS or GPS, they don't help ATC. You have to give ATC the actual position reference in the Route section of your flight plan. The most common formats would be:
Latitude/Longitude can be entered in either decimal deg or in deg min format. A decimal point is used for decimal degrees. An apostrophe is used for deg min. Use NSEW to identify directions.  They can be either prefix or suffix.  Be consistent.  For example:
N28.52W83.68  or  83.68W28.52N  or  N28'51'W83'40'
Place Bearing Distance and Place-Bearing/Place-Bearing waypoints start with a known database waypoint - typically a VOR - but any fix in the database will work. Examples:
ATL/162/18 to identify Atlanta VOR, 162 deg, 18 NM.
ORL70/MLB348 is the intersection of the Orlando VOR radial 070 and Melbourne VOR radial 348.


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What airfoils are used for the current generation of general aviation VTOL aircraft?
What airfoils are used for VTOL ultralights at 60-70 deg AOA without flaps or slats?
An example of this would be the Blackfly with a takeoff distance of 3', about 25% of it's own length....!!!!
I understand most airfoils stall at about 15deg, slats and flaps allowing up to 30 deg AOA such as with the Zenith 701.
Yet the Blackfly has no slats or flaps and has controlled flight even at at 60,70,80,90 deg AOA.
I assume the distributed propulsion reduces the LOCAL AOA significantly, so that lift is only via the props from say 40-90 deg AOA, and then wings start to generate lift at about 40 deg AOA even without slats or flaps.
Yet, I was told distributed propulsion only works for low AOA. Very confused....
What airfoil type is used for this?
Would an airfoil like the Selig 1233 or Eppler 399 be appropriate for this?
I understand these are low Re airfoils, but these usually stall at a lower AOA than a classic airfoil like the NACA65018 used in the Zenith701.
Or do you need something very fat that will stall at a much higher AOA, like an airfoil with 25%-30% thickness?

I am at a loss about what you are asking. If you are wondering how does the Backfly fly at very high angles of attack, it is because it has a thrust to weight ratio higher than 1, clearly seen here

At that angle of attack a flat plate will work just as well as any airfoil.

Those airfoils never see angles of attack of more than a few degrees. Since both wings of the Blackfly carry propellers along their full span, their prop wash ensures that airflow is mostly along the wing's chord. Slow flight is not much different from the hover of quadcopters, in both cases the propellers create most if not all of the lift.

Blackfly ultralight (picture source)
So to answer your question directly: There are no VTOL ultralights where the wing is exposed to flow at 60-70° AoA. The airfoils used are most likely about 15% thick with moderate camber because they operate at single-digit angles of attack.

Oh. So that means that these wings will never stall as long as the props are spinning, and they will create lift at 40,50,60,70 deg wing AOA ( but under 10 deg local AOA). Nice.


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How is the air traffic managed at a field with many privately owned aircraft?
As example "Spruce Creek". Many residents own private aircraft and can fly anytime they want. Do they have an separate air traffic controller? How is this high amount of air traffic managed?

While a fairly busy GA airport, Spruce Creek only has around 70 aircraft movements per day - that's less than Heathrow has in an hour. It's really not that big a deal

How is this high amount of air traffic managed?

Air traffic is managed at many levels by many people and is heavily dependent on the airports specifics when it comes to GA. First off not every airplane in the sky is talking to or under control of ATC, for that mater in some airspaces you don't even need a radio on board. The following is pursuant to US FAA regulations, other countries may handle air traffic differently.
The first level of traffic management is the pilot, its the job of every pilot to "see and avoid" other traffic, this goes for both commercial and general aviation traffic. Even if issued a command by ATC you can deviate to avoid an emergency (i.e. hitting another plane).
The next level of traffic management is the various FAA regulations on both ground and air flight operations. These regulations (like traffic rules) are laws and must be followed. They are designed to mitigate traffic in certain areas and will also force you in some areas to at least be on the radio with ATC (e.g. class B airspace). You can read up on the full regs here and you will find most of what you are looking for under "flight rules". Much of the country (below 18,000Ft and above 1200ft generally) is actually class E airspace which does not require ATC communications. This is where most GA traffic lives although small turbo props and the boldest of piston singles can now make it into the levels, most GA planes cant. There are also pre set cruising altitude intervals for VFR and IFR traffic to help keep separation.
Above that you will have ATC who is controlling all IFR traffic, traffic in Class A, B, C, and D airspace and will provide limited radar services to VFR traffic when load permits. When flying under VFR (depending on where you are) you may not need to ever talk to ATC. It is possible to depart an un-towered airport in class G or E airspace, fly around and return with out ever having contacted an ATC facility.
On the ground control depends on the airport. At an uncontrolled field (lots of smaller general aviation airports) the pilots decide. Taxing around is much like driving a car and there are pre determined radio calls to enter the traffic pattern and use the active runway. At a towered airport ground control will monitor and control the flow of traffic on the ramp and the tower will control the traffic from the runway thresholds until the plane departs its airspace.
The only exceptions to the rules here in the US are 5 airports (LaGuardia, Newark,  O'Hare, Ronald Reagan National, and John F. Kennedy) which are subject to the FAA's high density airport rules (specific to the airports) generally speaking the operations are the same in terms of how ATC will handle a given aircraft but departures and arrivals are limited and must be reserved ahead of time.
Another note on IFR operations, ATC will treat all planes the same and will only give priority to a plane in an emergency. Your little Cessna 172 on an IFR flight will receive the same services as a 747 30,000ft above you.

Do they have an separate air traffic controller?

No. See above but for what its worth the big planes contact the same ATC facilities as the little guys.
Spruce Creek
For what its worth Spruce Creek sits under a Class C shelf as you can see on the sectional here.

Departing from RW 5 and staying below 1200 puts you right between the Daytona Class C and New Smyrna Beach Class D so you may find your self contacting ATC on departure depending on your route you are somewhat boxed in there. Even departing from 23 you may find your self contacting Daytona approach to punch into the Class C there. Technically you can get out of there with out contacting ATC but you may find your self needing to depending on the situation.

At untowered airfields, like Spruce Creek there is no "ATC," the pilots self organize.
Generally speaking, the way this works is that as you approach you announce your arrival on the common frequency used by the airfield, 122.975 in the case of Spruce Creek. This frequency is listed both on aeronautical charts and in the FAA airport directory. So, for example, you might say something like this:
Spruce Creek, November niner seven six five four, Cessna one seven two, three miles out on one eight zero, request runway, Spruce Creek
Other pilots at the airfield will hear you and react appropriately. As you get closer you repeat your announcement and intentions. For example, the pilot might say:
Spruce Creek, November niner seven entering downwind for two three, traffic in sight, Spruce Creek
If the traffic is so heavy that you cannot enter the landing pattern, then you bear off and do a circle or racetrack pattern away from the airfield and try to approach again later. Often registrations will be abbreviated in various ways. For example, instead of saying "November niner seven six five four" every time, if you are the only Cessna, you can just say "Cessna", and the other pilot, say it is an Archer or Cherokee, will just say "Archer" or "Cherokee".

ATC is responsible for all aircraft, not only commercial IFR, but also general aviation IFR and VFR. Some airports may have restrictions (EDDM - Munich does not accept VFR below 2to MTOM), but in most other cases VFR general aviation is sequenced between IFR commercial.
Enroute VFR traffic may not necessarily be on the same frequencies, some countries offer own Flight Information Services for VFR traffic, where separation is not required, some airspaces that are shared require separation and will have both aircraft on the same frequency.
In the case of Spruce Creeks, there is a listed CTAF frequency, on which aircraft will be in the vicinity of the airfield. Leaving the airfield vicinity, they will join the regular frequencies for the services they require enroute.

Do they need to get their  flight plans submitted before flying and i think they must pay for the ATC services as well??

@NitinG It's entirely possible that they are paying a flat fee for that airport services. And flight plans are not required for everything, a local VFR flight might possibly be conducted without filing a flight plan. Depends on the airspace structure and jurisdiction.

@NitinG ATC services in the US are paid for by general taxation, there are no direct fees (usually called "user fees") like there are in many other countries. But there are often airport fees (landing, parking, hangars etc.) to pay, and I assume that in a private community like Spruce Creek the owners would pay for general maintenance too.

"ATC services in the US are paid for by general taxation"
Partly that, but also in large part by fuel tax on each gallon of fuel purchased.
Small local airports, like mine in Massachusetts, charge monthly rent for a parking spot (tie-down outside) or a higher rent for a lockable hangar. Fuel and airplane maintenance is on top of that of course. No ATC, we self announce and practice See and Avoid. Mandatory ADBS-Out is required for all plane by year 2020 (there might be some small number of exceptions) and planes with ADSB-In will have a display to provide position alerts of the nearby ADSB-Out


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How do combat jets know when to launch flares to counter IR missiles?
From my related question, it seems fighter jets do not usually track missiles by radar. Instead, they have a system that warns of radar lock.
So what about infrared missiles (heat-seeking missiles)? Flares are a countermeasure to these, so how does the jet know when one is incoming?
I would be pretty surprised if such a system didn't exist. I want to know more about that system and when it was first implemented.

Actually most aircraft don't actively know when a missile is being fired at them. I have worked on helicopters and fighters that deploy counter measures.
The first is an IR jammer which we call a disco ball. It is made up of mirrors at different angles that resemble a disco ball. It radiates an IR signal at different angles to confuse the missiles.
The flares or chaff are fired periodically in known hot spots. This creates heat signatures similar to the engines and confuses the missiles.
There are active systems but they don't work well. I have seen a helicopter in testing suspended from a wire with countermeasures turned on. A missile was fired and the countermeasures didn't even get deployed. This is why they usually keep firing the flares or chaff in known hot spots.
Hope that helps.

How many flares could fit on a fighter jet or helicopter? It sounds to me like this approach would quickly run out of flares.

From memory on a helicopter, the left side has a self defence suite with 4 rows of 6 tubed.  So 24 on each side, up to 48. But don't quote me on the number. If you ever watch the news with a fighter jet in Afghanistan flying over a hot spot they continously fire flares every 2 or 3 seconds. Contrary to what movies display, aircraft don't stay in hot zones for a long time for obvious reasons

You should be able to Google some pics. The boxes will be just behind the doors above the skids. Just count the tubed.  No secrets there.

How disco ball reflects IR signal? AFAIK IR signal is just heat.

With mirrors. Here is a link on the specific system I worked on. https://en.m.wikipedia.org/wiki/AN/ALQ-144

@Mark I recommend that you remove the last comment.  Asking for upvotes is seen as impolite (at best) and many people who might otherwise upvote won't when they see your comment. Additionally, your answer is wrong.

@Mark I don't know how you got the idea that disco balls "reflects the missiles IR signal".  IR missiles don't transmit anything, therefore there is nothing to reflect.  Disco balls work by radiating large amounts of IR pulsed and modulated to confuse a missile.

I upvoted because this poster seems to have experience working in this field and can give us firsthand accounts. (Give us some time to upvote, we don't live on this site 24/7, at least not always :) @Simon I don't see anything wrong per se, but the disco ball could use better explanation. Correct me if I'm wrong but I think what he's trying to say is that it generates an IR signature somewhere other than the engines or center of the aircraft, so that when the missile leads its target (as it should), it is leading off center and will miss.

@DrZ214 Nope. Disco ball jammers (that's a bad term) aim to flood the receiver on the missile making it break lock. The acquisition beam of a most missiles is very narrow, a few degrees, and once lock is broken, it is very unlikely to re-acquire. The missile will simply fall away. It's too complex to detail here but in essence, the missile scanning head sweeps from side to side to track the heat source.  Disco balls emit a large source of modulated IR which floods the receiver and prevents it from tracking.  Modern missiles render this obsolete and disco balls actually attract them.

@DrZ214 Most IR missiles aim slightly off centre deliberately and explode alongside the target using proximity fuses.  Some fire out rods or chains which rip through the aircraft letting holed fuel tanks and engines disintegrating do the real work.  The aim of passive IR jammers is to stop lock.

Sorry I misspoke, was late at night. The IR signal is transmitted from the ball itself confusing the missile. The disco ball or mirrors scatter the signal.

Also note that chaff (or "window") is an anti-RADAR counter measure and is not effective against IR seeking.  I would edit it out but you might have had something else to add?

@simon - The chaff can catch sunlight and engine heat and add noise to the missile's seeker picture, but on the whole you're right, it's not very effective against IR missiles. It was originally designed in WWII to confuse German ground radar sites, an early form of jamming.

@KeithS If you hold a flashlight up the the sun, are you going to see the light of the flashlight, or the light of the sun? There's almost no chance any IR missile is going to guide on chaff when the engine is vastly overpowering its heat signature.

@Simon Typically when countermeasures are fired, they are fired in a 'program' that fires both chaff and flare at the same time in a pre-determined mix. There are two types of countermeasures on the plane, but only one countermeasure system that can be fired. These programs are usually set up before the flight, and optimized against specific threats.

Before continuing, much of the how's and why's of IRCM are going to be either speculation, or classified.  Nations will not freely discuss their actual methods of defeating opposing nation's IR capabilities.  Therefore, actual answers will be difficult to give.  However, here is an excerpt from Wikipedia regarding a specific missile warning system that may be found on military aircraft.  This represents one possible method used for certain types of aircraft.

The AN/AAR-47 Missile Warning System is a Missile Approach Warning system used on slow moving aircraft such as helicopters and military transport aircraft to notify the pilot of threats and to trigger the aircraft's countermeasures systems.

The AN/AAR-47 passively detects missiles by their infrared signature, and uses algorithms to differentiate between incoming missiles and false alarms."
https://en.wikipedia.org/wiki/AN/AAR-47_Missile_Approach_Warning_System

The infrared missiles are usually detected using a Missile Approach Warning Sensor (MAWS).
The Missile Approach Warning Systems are passive detectors, usually detecting either the infrared or ultraviolet rays emitted by the incoming missiles.
The infrared based systems detect the infrared waves emitted by the missile. The ultraviolet based systems detect the ultraviolet rays emitted by the missile's rocket motors and are more suitable for detecting missiles with solid rocket motors.
An example for the infrared based detector is the US-Israel PAWS, while the Swedish MAW-300 uses an ultraviolet detector. Some systems like the MWS-20 uses Doppler Radar to detect the missiles.
All these systems have their advantages and disadvantages, and in relation to your previous question, give a 360 degree coverage based on the location of their installation.
In most of the aircraft, these detection systems, along with the Radar Warning Receiver, which detect the Radar based missiles are integrated into the Self Protection System(SPS), which determines the threat level of the incoming missile and responds accordingly by firing chaff and flares.

I went to Wikipedia to cite that IR missiles are entirely passive and I found to my surprise that the AIM-9 has four IR emitters! +1

@ToddWilcox Those emitters are only on some models with IR proximity fuses.  They have nothing to do with acquisition and tracking and are only used to detonate the warhead when alongside the target.

Equally worth noting, some Air-to-Air IR missiles (like some variants of the AIM-9) can be given their initial steering cues via the FCR on the aircraft, this is active radar illumination on the target and can alert the pilot, causing him to trigger flares manually, even though the rocket and guidance system being used are IR. To say nothing of the fact that sometimes flares are triggered simply because the pilot sees the missile launch.

@Mark How long does a flare usually last? I want to get an idea of how many might really be needed for extended dogfights.

@DrZ214 The idea of an extended dogfight is somewhat far-fetched. If you're talking specifically about a 1v1, then you shouldn't be looking at how long the flare lasts but how many missiles you will need to duck. Airplane loadouts are extremely small - the F-16 can only carry 6 air to air missiles, the F-15 can only carry 8. If you assume something like 4 flares per missile, you're only look at 24-32 flares (assuming you are only optimizing for a single operation of 1v1 dogfight). You'll hit fuel limits well before you hit weapons or countermeasure limits, though.

@ToddWilcox: Air friction is going to heat up the nose cone of a missile. It may not be an intentional IR emitter, but it will show up.

@MSalters  I would hate to have my life relying on that being picked up against background clutter. I would have through that even from head-on, the scattered IR from the rocket motor would drown out the friction signature?

@MikeBrockington: AFAIK, that may not be a major source in mid-flight.


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What data did the earliest HUDs display (for fighter jets)?
Ideally I'm looking for a screenshot with a bullet list of data. I started my search at the obvious encyclopedic site and was surprised to learn that the earliest HUDs (on fighter jets) go all the way back to the 40's.
So what data did they display? Presumably it would be a limited set of data compared to today's standards, and I'm very interested in what was considered the most important data for a HUD. Especially, did they have that circular piper showing where your bullets would impact?

Looks like the AI Mk VIII radar on the Mosquito had an artificial horizon plus gunsight (and target marker), hard to find any illustrations or photos though.

@egid but no WW2 era fighters ever had a HUD, did they? What year did this feature get added to the Mosquito?

this post on a forum suggests that it was tested in 1942 but not adopted for widespread use. Illustrations included!

@DrZ214, what WW2 era fighters did have was ‘parallax gunsights’ that work basically the same way as HUD but only project static image, possibly adjusted by gyroscope to provide correction for own turn. The advantage was that the gunsight aligned even when pilot moved their head.

"Ideally I'm looking for a screenshot with a bulle tlist of data" - Are you asking us to do your research for you?  The question is fine but that one phrase does stick out to me.

Reflector Sight:
The Me 262 Schwalbe, introduced in 1944 as the world's first operational jet fighter aircraft, did not have a full-blown HUD. Instead, like many fighters of WWII, it had a reflector sight which used lenses and reflecting glass to display a reticle. This sort of thing [all of the bright yellow lines you see in the image are fixed together. Although you only really needed a dot and a circle (like the Spitfire), some pilots liked the extra lines to help line up targets that were not dead-ahead. They were favored by pilots over iron-sights because the pilot was not required to position his head in order to see where the guns would fire.
Simple HUD
The MiG-21, introduced in 1959, used a combination of reflector sight and HUD. Though it didn't display the altitude, speed, heading, or attitude in a way that modern fighter pilots may take for granted, it had some ability to track targets with radar and to display predicted target positions for gun or missile use. Here is a guide on how to aim your guns using its sight. Also, explanations of the displays. The following image was posted on a forum thread related to the MiG-23, which came after the MiG-21, but this image looks much like those from the MiG-21 manual. A different image from another author shows you how simple the display could get. Finally, an F-16 in the sights of a MiG-21. You can tell this is a MiG because of the distinctive pitot tube.
Full HUD
The F-14 first flew in 1970. Its HUD was equipped with an artificial horizon, altitude, speed, heading, gun data, missile data, and much more. This is from the F/A-18, which first flew almost a decade later.

Thanks for the data but can you post a source for this, especially the MiG-21 HUD? Why does that image show data in English instead of Russian? Also, are you sure the first MiG-21's had this in 1959? Or was it only later versions?

@DrZ214 My mistake, that was a much later, upgraded version of the MiG-21 (Lancer, a modernized development). I have added more information (and more links!)

As an aside, the F-14's HUD features should be familiar to many GA pilots that have flown a glass cockpit; the basic layout of military HUDs, minus targeting displays, is the basis for modern PFDs, the only real difference being the multicolor presentation instead of a monochrome projection onto a reflector.

The Me-262 Reflexvisier was special because it was the first (for the Luftwaffe) with gyroscopic control to predict where to shoot in a dogfight. However, it was so complicated to adjust that many ground crew simply fixed it in the straight forward position (which mattered less than you might think, because the 262 was no great dogfighter …)

The earliest were reflector sights which were fitted to fighter aircraft at the end of WW I. The picture below shows an Oigee Reflexvisier from 1918, built at the Optische Anstalt Oigee factory in Berlin-Charlottenburg and used that year in Albatros, Fokker and Pfalz D-type aircraft. Two types were built and were the first, albeit very simple, head-up displays.

Oigee Reflexvisier (picture source)
What was displayed was basically a single dot of light which was projected on the semi-transparent mirror such that it would overlay the point at which the fixed guns were aiming, regardless of the pilot's eye position. Only the height of the mirror needed to be adjusted for the sitting height of the pilot. The brown plate in the path of light was used for this adjustment and removed for operation.

Computing (Gyro) Gunsight
As a development of the existing "reflector gunsight" (which as other answers note was intended to eliminate parallax error due to the pilot's head position), the "computing gunsight" used the motion of the aircraft it was mounted on to generate a modified gunsight indicating the lead required to shoot a moving target.
A ring was projected onto the reflector, such that holding the target steady in this ring would provide the correct lead.  An estimate of the target's range first had to be set on the gunsight.  In practice, this was often left at the convergence range of the guns, and the pilot would attempt to manoeuvre the target to that range.
The first production models were produced in Britain in 1941, and successfully used against the Luftwaffe.  In 1942, a Thunderbolt with a computing gunsight was captured in Germany, which stimulated their efforts to develop similar technology.
In this early technology, no corresponding information about altitude, airspeed, heading or attitude were provided, being left to conventional instruments.  The Sperry K-14 was considered notable for including both computing and fixed (ie. equivalent to a reflector sight) gunsights in the same unit.


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Why are HUDs usually green instead of red?
Every HUD I've seen, whether photographs or in video games, uses green light to draw the lines and text.
Why green? I thought red was a better color for pilots' eyes so they can stay adjusted for night vision. Do HUD's allow you to switch colors at night?
Note: I'm interested in military aircraft only, especially fighter jets. But if the reason happens to be the same across the military and civilian world, that's fine too of course.

Head up displays offer colors in a wide range of hues to fit different lighting conditions, though green is the one most often used for the reasons below.   The display color is adjustable via a dial on the hood controller.

A few thoughts (a bit of personal research):
Eyesight

The eye is most sensitive to green color, as can be seen in this diagram:

At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength.

(source: wikimedia.org)
Dotted R line is (night-vision) human rhodopsin 'rods'.
As I understand, any powerful light depletes rhodopsin down requiring that they regenerate, which takes time, just that depletion takes longer with red wavelengths. I think this does not imply that green color 'automatically' screw up night vision if the display intensity is lowered enough.
Technical Limitations

The light has to be collimated to not interfere with depth perception, which complicates having multiple colours. At least in old CRT displays, this would have left you with the choice between blue, red or green. Green and Red is perhaps the color which stands out the most, but this diagram would suggest the red occurs at different wavelengths, perhaps again interfering with collimation:

Due to the complexity of wide field-of-view reflective HUD optical systems, the optical designer must use all means available to meet display accuracy and parallax error requirements. All certified reflective HUDs today are monochromatic, generally using a narrow-band green emitting phosphor. The addition of a second color to the HUD is a desirable natural progression in HUD technology, however, one of the
  technical challenges associated with adding a second (or third) display color is maintaining the performance standards available in monochrome displays. One method for solving this problem uses a collimator with two independent embedded curvatures, one optimized for green symbology, the other optimized for red symbology, each with a wavelength-selective coating. Avionics Handbook, Chapter 4

Compounding to the previous point, it may be difficult to filter through a single red color through the screen, ending with a rather broad spectrum instead:

Further thoughts:

I think the night vision would be disturbed anyway by the variety of instruments such as maps and radar. I'm also doubting a bit of the importance of night vision in combat situations.
Night vision googles also use monochrome green light, despite having perhaps the greatest benefit of red light should they break and you need to continue without.
Even in a pretty new BMW I drove there was only monochrome green from their own HUD. I read forums accusing systems featuring multiple colours as having inferior display quality.

Perhaps you could add a remark on why white is not considered as a HUD color in aviation - even though white LEDs are currently widely available. (I used one on a BMW the other day, white color, and had difficulty reading the display well in certain combinations of lighting and background.)

@ALANWARD With white you got the full color spectrum and wavelengths (LED example) probably screwing up the colliminating again. Furthermore, contrast against clouds and skies is not good.

My feeling in the car was indeed mostly about lack of contrast. Nothing scientific, but a definite feel that something was not right.

Yeah, broad-spectrum (white) light suffers chromatic aberration when passed through any optical system: for an example, look at the blue or brown fringes you get from an overhead projector.

Another thing to consider is that, in the case of the military, green lights work best because they are NVG compatible.  NVG's have a hard time with red lights because NVG's operate in the near infrared spectrum causing red lights to show up much brighter and potentially bloom the display.

@ALANWARD, note, that white LED are a bit of a cheat. LEDs naturally emit monochromatic light and the white ones have a luniophore that spreads out the emitted spectrum. And even then they usually have two distinct peaks, one narrow blue from the LED itself and one wider yellow-green from the luminophore, that together are perceived as white, but because they don't really cover the full spectrum, they still skew colours of objects quite a bit.

Studies show that our eyes see green better than any other color in both day and night. There is an article about the difference between green and red lasers on guns and how you can actually seen green better in day and night so that's why the color is green.

From this gallery is a picture of a HUD -- look at how visible and bright it is.


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How can I improve the reception of my coaxial collinear antenna (1090 MHz)?
I've built a coaxial collinear antenna (16 arrays), according to this guide but having some issues picking up traffic from the West (?). I'm not very experienced in the area, so I've no idea where to start or why this is happening.
The antenna is located in my living room and is aprox. 2 meters.

Might it have something to do with this: http://aviation.stackexchange.com/questions/2345/relation-between-aircraft-altitude-and-ads-b-coverage/2346#2346 or does that not matter much, if we're only talking about ~50km distances.

Sounds like more of an antenna design question, therefore physics.SE or amateur radio. The diagrams on the site appear to be fairly directional, and it depends on how you have the antenna mounted.

@fooot You might be right in regards to the directional - I might just drive to the airport and look at the signal there.. see what happens. Thanks a lot for your reply :-)

What receiver do you use?

@MasterYoda Do you get better results when the antenna is outside? I am interested to see how well such a 16 element colinear performs.

Hi @DeltaLima. I'm using a R820T dongle; I tested the antenna outside, and I get aprox. 130km range, whereas the antenna that shipped with the dongle, gets aprox. 30km - so it's a pretty big improvement. I use 75 ohm cables. It's a pretty simple build, takes about 30 min .. for the amount of gain vs. the low cost, you should definitely go for it.

Not too bad considering the minor investment. I am used to a bit longer ranges but the equipment I use to is a bit more expensive as well.

If the antenna is located in your living room, then the incoming 1090 MHz signals will be blocked by your house. 1090MHz signals are more-or-less line of sight. They don't travel very far through concrete or wood and are severely attenuated by glass, especially is you have energy efficient windows.
I suspect you have a window towards the East or South East so that you have some signal reception from that direction. Try to put the antenna outside, preferably on top of your house to improve your signal reception.

Hi @DeltaLima - this helped a lot, so you're right in regards to the directional issue :-).


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What level of G-force does our body experience when we are traveling in a passenger plane?
I have some understanding on g-force. But never come across how much g-force acts on a passenger who is traveling in a plane, especially when taking off.

Related: http://aviation.stackexchange.com/questions/2671/why-do-i-get-that-funny-stomach-drop-feeling-right-after-takeoff

There are two basic classes of G-load; static and dynamic. The difference is time; static loads are those placed on the airframe and its contents during relatively sustained maneuvers like climbing, banked turns, etc. Dynamic loads are transient; turbulence and touchdown are the two big ones. Which one we're talking about is very germane to the question; most maneuvers deal primarily with a static load caused by the change in direction of travel over a sustained turn or pitch-up, however dynamic loads from turbulence while maneuvering must also be accounted for.
Also very relevant is the weight of the aircraft at the time of a g-loading event; g-forces are always a multiple of this mass relative to the force of gravity on that mass. The weight used for rating purposes is usually the aircraft's max take-off weight, because this is the heaviest that a plane is expected to be while flying (and thus subject to maneuvering loads), and so the most pessimistic.
For static G-forces, the 737 class operating G-range is between +2.5g to -1.0g at MTOW in "clean" configuration (reduced to between 2.0-0.0g under full flap). Multiplied by the 737-800's MTOW of 79,000 kg (174,165 lbs), we get a critical number when talking about such things, which is known as the "design load limit", which for the 738 is 197,500 kg (435,412 lbs). The aircraft's combination of mass and G-loading should not exceed this number.
This means the aircraft can, theoretically, withstand much higher g-forces on a ferry flight when there are no passengers or cargo; at the 738's operating empty weight of 41,145 kg (90,710 lbs), the DLL represents a maneuvering of 4.8g.
You are unlikely ever to get anywhere close on the average passenger flight in a sustained maneuver; such a banked turn would be hard even for the pilots to sustain. A "steep turn", using wing bank in the 20-40* range, is about the steepest maneuver you're likely to experience on a commercial airliner, when ATC asks for a "nonstandard turn" during approach or departure to fit the plane through a gap in traffic. Even in this maneuver, you're unlikely to feel more than about 1.25g of force pushing you more firmly in your seat.
The aircraft, however, can take even more. The DLL is the "safe working load limit" of the aircraft under normal conditions, which pilots and airlines shouldn't exceed. To prove the airframe can handle that load, every flying airframe is "proofed" to 1.125x the DLL (representing a 2.8g load at MTOW) which it must survive without structural compromise ("bending"). A sacrificial example of the airframe is also failure-tested to the "Design Ultimate Load", a static load of 1.5 times the DLL (representing a 3.75g load at MTOW); it must withstand at least this load for 3 seconds without structural failure ("breaking").
FAA regs (Title 14 Part 25) state that any "Transport"-category aircraft (encompassing virtually all commercial airliners) must be demonstrated to withstand a loading of at least 2.5g at MTOW, but do not have to be able to withstand more than 3.8g at MTOW. In addition, there is a general requirement that the aircraft must be able to withstand any vibration, buffeting or turbulent forces that could occur at any speed up to the "design diving speed" VD, and must withstand any additional forces that could be imparted to the aircraft by a failure or malfunction of any part of the flight control system at speeds up to the airframe's "design cruising speed", VC. So, all airliners from the little Bombardier narrow-bodies up to the A380 have to be able to at least match the 737 in their g-loading capabilities.

This is a great answer to another question but does not answer the OP who may well get the impression that Gs in excess 1.5 are routine. In normal ops, passengers are going to feel nowhere near the limits you quote.

I can't find solid references, hopefully someone will come along who can.
A 30 degree bank, the most you will normally experience, exerts a force of 1.15g, or 0.15 greater than normal acceleration you feel when motionless.  The autopilot is generally designed to begin descents and climbs with no more than 0.25g and are generally calibrated to manoeuvre in 0.25g increments.  A solid rotate or a firm landing might generate a bit more.
It's reasonable to work on a range of around 0.75 to 1.25gs. Anything more than that would be uncomfortable for many people.  You should only experience g outside that sort of range in heavy or severe turbulence.

The forward and backward forces created by the engines during takeoff roll and the brakes/reverse thrust during the landing rollout are also quite non-trivial. Even at MTOW, a 753 can produce around 0.32 g acceleration during takeoff (and probably more on landing.) A light 753 could accelerate at around .6 g.

@reirab Is this one of the reasons pilots don't use max thrust on takeoff?

@yo', no, it isn't. Forward acceleration is no problem when everybody is sitting facing forward. Engine wear is the only reason.

@Simon for the "firm landing", what are the values?

@ManuH That's going to depend on the aircraft and the weight but from memory, a really hard one is 2.25g.  That would almost certainly cause damage.  I believe that a normal "firm" landing is around 1.8g. We could do with Terry.


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Why do the Lycoming HIO-360 A1A and IO-360 engines produce different power?
I have just acquired an aerobatic aircraft fitted with a Lycoming HIO-360 A1A. Reading the specs on the engine I see it has the higher (8.7:1) compression of the 200hp IO-360 engines, which seem mainly to make that power at 2 700 rpm, but this one actually makes the same power (180 hp) of the 8.5:1 carburetter engines, but at higher rpm (2 900) than they turn. This was a helicopter engine originally which I think accounts for the higher rpm rating, but it seems to me wholly counter-intuitive that an engine with the same swept volume, fuel injection for greater efficiency AND more revs would make - relatively - so little power. Can anyone explain this?

There's a lot more to engine design than RPM, compression ratio and volume. I guess this an engine optimized for fuel savings and lower temperatures since helicopters may not have the continuous forwards airflow for cooling, and perhaps it's more capable of running at high RPM for prolonged periods of time without wearing the engine.

Those power rating explanations in other answers are incorrect.
The Helicopter engines are rated at a lower horsepower at a higher RPM because they cannot accept the SAE "5% less than rated is acceptable" rule.
The power rating must be correct, and is set at an altitude above sea level, for example, 1500' ASL. This is easily understood when you look at the engine operator's manual.
I have an IVO-360A1A from a Brantley B2B Helicopter modified to fit in my RV-8. It's a parallel valve engine, 8.5:1 compression ratio, rated at 180 HP @ 2900 RPM. It has a TBO of 1000 hours. It had a huge oil cooler and it also has oil lines that extend from the oil galleries on the engine block to the exhaust valve guides, where oil flows into the head, around the outside of the exhaust valve guide and drains into the valve chest to cool the head. That is because helicopter engines work very hard and run very hot. Internally they are nearly identical to aircraft engines, and use the same parts.

You can improve your answer by adding references and links to provide further readings.

SAE has nothing to do with the rated power of an FAA certified engine. See answer below.

Helicopter engines have different operating parameters than aircraft engines. They operate in a narrower power band and the difference in horsepower would be due to  camshaft lift and duration optimized for that power band.

While engines of the same size may have different ratings for a host of reasons, the FAA rules (14CFR33) for rating the power of a helicopter engine are much more demanding than those for rating an airplane engine, and it has nothing to do with SAE.
Specifically, the endurance test for certifying rated power is more stringent. Both sets of rules require 150 hours of sustained running (servicing allowed) and alternate between periods (mostly 1.5-2.5hr) of rated speed/power and reduced speed/power.

The average reduced power for helicopters is 71%, for airplanes it is 64%
The average reduced speed for helicopters is 71%, for airplanes it is 84%
The speed range for helicopters is 30% to 110%, for airplanes it is 79.5% to 100%
The helicopter rules require a sustained 15hr run at 105% of continuous rated power and speed, and has 12.5hrs at 110% speed. There is no similar requirement for airplane engines.
An airplane engine spends half of its time at reduced speed, a helicopter engine spends 15hrs more at rated power than reduced power.

Helicopters must produce more power under a wider variety of more difficult test conditions: 71% power at 71% speed is 24% harder on the engine than 64% power at 84% speed. Consequently, a helicopter engine that is internally exactly the same as an airplane engine will be rated 5-10% lower in power at 5-10% higher in speed. The helicopter version of similarly rated engines will be beefier than the airplane version.

The Lycoming HIO-360 is a horizontally-mounted version of the IO-360.  (H for Horizontal).  It reaches its power band peak and has a lower peak at different RPM for that reason.  This is often the case when you work against gravity / work with gravity.
Some helicopter engines are designed with thinner cylinder walls for lighter weight. (IO-360-J2A for the Robinson R22 for example).  These are further derated because of the weaker cylinder walls.  Note that there is no H designation here because a)H is for horizontal and this is not one, and b)H is NOT for helicopter :)
E

The "H" in HIO-360 is really not for "horizontal". All variants of the 0-360 are mounted horizontally. The "H" is for helicopter version of a horizontally mounted O-360. The O-540 series are mostly mounted horizontally, but they also have HIO-540 (horizontally mounted helicpoter) and IVO-540 (vertically mounted helicopter) versions. https://en.wikipedia.org/wiki/Lycoming_Engines

Reciprocating light helicopter engines are derated to allow for out of ground effect (OGE) hover at an altitude above sea level. To the best of my recollection the R22 is limited to a manifold pressure of 25 inches. This allows for rated power to be achievable at roughly 5000 ft. altitude.


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Have there been any fatal accidents involving a transatlantic flight since AF447?
I'm trying to confirm there has been no commercial airline transatlantic flight accident (with fatalities) since AF447 in 2009. This would include any flight that crossed the Atlantic ocean, even if the incident was on landing. I believe this is the case as of November 2021, but would like to be sure.

What are we trying to answer here? Dropping into the ocean (ala AF447) or something which has made a transatlantic flight and crashed on landing?

Are you looking for just airline flights? There are lots of transatlantic flights that are not airlines, such as military, research (NOAA, etc.), and even ferry flights of small, single-engine planes.

Does "transatlantic" mean anything that transits over the Atlantic ocean, or does it have to go to/from Europe/Asia/Africa to the Americas?

@FredLarson I did actually specify commercial, but Pondlife completely rephrased the question and removed important info

@Cloud I'm not sure which edit you mean. I changed the title only; another user changed the question body. In any case, it's your question and if edits aren't helpful then you should absolutely roll back or edit again!

If this counts (the planned route of flight included an Atlantic transit)...
Fly Jamaica Airways 256 in 2018
The aircraft was scheduled to fly from Guyana to Canada but experienced a hydraulic failure on the climb out, returned to the airport and overran the runway hitting a perimeter fence on landing.
Six people were injured and an 86 year old woman died of injuries 7 days later, so it is classified as a fatal accident.

I'm not sure such route fully qualifies for transatlantic (which I associate to crossing the ocean) but you asked for details and were not answered, so it's fair. And it's indeed a very good find. +1


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Is there a correlation between airport capacity and accident rates?
This question about the new Heathrow runway increasing flights per day by 700 mentions that this airport (and many others around the world) runs at over 98% capacity.
This got me thinking, is there a correlation between airport capacity and accident rates?
One would expect that the airports running at the highest capacity, would have the highest rate of fatalities per million flights, or whatever metric you want to use. But I am interested to know if this is the case or not.
*Only interested, for statistical reasons, in incidents that occurred within 5 minutes of take-off or on approach.

Does this question mean anything? At what point can we separate an air crash from an airport? The two most high-profile crashes in recent months are suspected to have occurred due to a fault in flight control software. Are those crashes related to the airports they departed from? A 777 crashed landing at Heathrow because of ice crystals in the fuel. Is that related to the originating airport, or the destination? The premise is insufficiently well defined to be answerable.

Fatalities are unusual at airports, mostly they occur en route

@DaveGremlin Updated question

"One would expect that the airports running at the highest capacity, would have the highest rate of fatalities per million flights" Why would one expect that? I can't think of any logical reason as to why that would be the case

@J.Hougaard The same reason the busiest roads have the highest rate of fatalities

@Cloud you appear to be confusing number vs rate. More traffic may mean more accidents but not a high rate per 1000 vehicle movements.

@Notts90 Nope, I know the difference very well and my statement still applies.

@Cloud I think you need some citations to backup your claims.

@Notts90 No I don't, since my question doesn't pertain to automobile statistics in any way

@Cloud So why is the fatality rate high on busy roads? Because cars sometimes hit each other, and more cars mean more collisions. Planes, however, do not hit each other. When planes crash, it happens for other reasons (mechanical failure, human error etc.) - and there is no logical reason to assume that the rate of such occurrences is higher at busier airports.

I think the problem with this question is that it conflates incidents, accidents and fatalites.  It's almost inevitable that there will be more incidents such as equipment failures because there are more aircraft around, accidents involving damage and/or injuries seem to correlate more to which country you are in (judging from a quick glance at AVHerald), and fatalities are very unusual at airports

An airport which is at 100% capacity with one flight per week (for whatever hypothetical restriction) is less likely to see an accident than an airport running at 20% capacity with 5 movements per hour 24/7. So I don’t see why one would necessarily suspect a relation between capacity and accident rate without further explanation.

The factor that has been overlooked is type of operation. You need to normalize by this. General and corporate aviation have a higher accident rate than air carriers. Training is likely the worst. Large airports discourage such operations, or even ban them outright. You aren't going to have student pilots flying piston planes into LHR.

It is unreasonable to assume that a busier airport has a higher rate of fatalities compared to a smaller airport. There is just no logical reason to think so. The total number of fatalities may well be higher simply because there is a larger number of flights, but I know that is not what you are asking.
You might think that busier airports would have more plane accidents the same way that busier roads have more car accidents. This comparison is flawed. The reason busy roads have more accidents is because more cars lead to more collisions. However, with very, very, very few exceptions, airplanes simply do not collide, especially not at busy airports where the air traffic is managed by air traffic control.
Additionally, larger airports typically have better equipped fire and rescue services, so if accidents occur, they are probably less likely to be fatal. Also, pilots flying to busy airports are typically experienced commercial pilots, since smaller private (leisure) flights are not really taking place at large airports.
I would expect the fatality rate to be much higher at a small, uncontrolled airfield with a short grass runway, no fire and rescue services and where the majority of flights are flown by relatively inexperienced private pilots operating under visual flight rules.

+1.  The last paragraph is particularly apposite.  The AAIB records show the most common form of accident is a bad landing, which also has the lowest and least serious injuries.  EFATOs are much more deadly but comparatively rare.  These figures reflect a much higher rate of accidents and incidents amongst private pilots